Multidrug resistant Gram-negative pathogens have been declared a leading, emerging health crisis by the CDC and WHO. Infections with these organisms, including Pseudomonas aeruginosa, Acinetobacter baumanni, and extended-spectrum beta-lactamase (ESBL) Enterobacteriaceae, carry ~60% increased mortality compared to infection with antibiotic-sensitive organism. Alarmingly, the prevalence of infection with resistant forms is steadily climbing, now >20% in some regions of the United States. In this setting, novel antibacterial agents are desperately needed. The most significant hurdle facing novel lead discovery against Gram-negative pathogens is poor drug permeation through the outer membrane, coupled with high rates of drug efflux due to redundant efflux pump systems, as it prevents intracellular drug accumulation and thus whole cell activity. Significant investments in target-based drug discovery efforts have identified many potent leads against conserved bacterial enzyme targets, but low intracellular drug concentrations in Gram-negative pathogens have doomed their development. These challenges in translating potent biochemical potency to effective cellular activity and thus in vivo activity without toxicity have been noted across the industry and uniformly resulted in strategic decisions to abandon this approach, despite the abundance of otherwise promising antibacterial leads. An approach to deliver such drug leads into the bacterial cytoplasm would be transforming, as it would leverage the tremendous investment that has already been made in the optimization of such leads. Further, a general platform for such delivery that could be applied to any such lead would transform the state of the antibiotic pipeline. We suggest that exploitation of native, active bacterial uptake systems is a potentially powerful strategy that can serve as a universal platform to deliver small molecules into the bacterial cytoplasm. By conjugating small molecule antibiotics that are potent for their cognate bacterial enzymatic target to the factor that is imported by these uptake systems, sufficient intracellular concentrations of the antibiotic can be achieved. We propose a novel system founded on the highly conserved and redundant Vitamin B12 uptake systems in P. aeruginosa. These systems efficiently transport cobalamin derivatives from the extracellular environment to the cytoplasm and are capable of transporting a range of cobalamin derivatives. We will develop optimized Vitamin B12-antibacterial conjugates that are programmed for optimal exposure and minimized toxicities from host cell uptake. In addition, we will leverage highly optimized, novel antibacterial drugs and leads whose barrier to development against Gram-negative bacteria is only cytoplasmic delivery as cargos. The successful delivery of this program will not only provide novel Gram-negative antibacterial agents poised for IND-enabling studies, but will also demonstrate the potential of active transport conjugate drug delivery approaches to transform antibacterial drug discovery.